1. Field of the Invention
The present invention relates to aqueous photosolder resist compositions which can be alkali developed, and also to solder resist films formed from the compositions.
2. Description of Related Art
Solder resist compositions are generally used in the form of liquid dispersions which are applied, in the form of coatings, onto substrates such as printed circuit boards. Images are transferred to the coatings by a sequence of light exposure and development. Their curing levels are further increased by the subsequent exposure to heat or light. The compositions for use in the formation of such solder resist films are typically solvent-borne. The use of a solvent is not desirable from an environmental point of view. This has led to the need to develop water-borne or aqueous solder resist compositions.
Japanese Patent Laying-Open No. Hei 5-100427 (1993) discloses active energy radiation-curable resins prepared by partially reacting xcex1-olefin having 6 or more carbon atoms with maleic anhydride, reacting the product with a compound having an unsaturated double bond and a hydroxyl group in a molecule, and then neutralizing the product with an alkaline substance to render it water-dispersible. It also discloses aqueous photosolder resist compositions containing a compound having at least one epoxy group.
Due to the incorporation of a glycidyl-form epoxy group which has a tendency to deactivate in an aqueous medium, the disclosed aqueous photosolder resist compositions suffer from the reduced stability when stored in the form of a liquid mixture containing a principal resin and a curing agent. This problem is attributed to the glycidyl group which is caused to react while the compositions are stored or dried. Its reaction also leads to reductions in various properties, e.g., reduces developability and heat resistance as a result of failure to form uniform films. Also, some compositions, in spite of their aqueous form, release solvent odors to adversely affect working environments. Other deficiencies include pattern defects after development and heat resistance reduction, as a result of insufficient dispersion stability of inorganic fillers.
It is an object of the present invention to provide an aqueous photosolder resist composition which is free from solvent odors and has satisfactory storage stability and excellent heat resistance.
The aqueous photosolder resist composition of the present invention contains:
(A) an aqueous solution prepared by neutralizing a resin containing a free-radically polymerizable group and a carboxyl group with a base;
(B) an inorganic filler;
(C) a photocurable mixture containing (c-1) a multifunctional acrylic monomer, (c-2) a compound having a cyclic ether group other than a glycidyl group and (c-3) a photoinitiator; and optionally
(D) an aqueous solution prepared by neutralizing a free-radically polymerized substance having an acid value of 130-230 mgKOH/mg with a base.
The aforementioned cyclic ether group other than a glycidyl group may be an alicyclic epoxy or oxetane group, for example. The aforementioned resin preferably has a free-radically polymerizable group content of 5xc3x9710xe2x88x924xe2x88x921xc3x9710xe2x88x922 mole/g and a carboxyl group content (i.e., acid value) of 30-200 mgKOH/g, more preferably of 30-130 mgKOH/g. The molar ratio of carboxyl groups present in the aqueous solutions (A) and (D) to cyclic ether groups, exclusive of glycidyl groups, present in the photocurable mixture (C) may be 1.3/0.7-0.7/1.3, for example.
The aqueous photosolder resist composition of the present invention may further contain a coloring pigment. The inorganic filler is contained preferably in the amount of 5-60 parts by weight, based on 100 parts by weight of solids in the aqueous photosolder resist composition. The solvent, if used, is contained preferably in the amount of not above 20 parts by weight, based on 100 parts by weight of solids in the composition.
The solder resist film of the present invention can be formed according to the following procedure: The aqueous photosolder resist composition of the present invention is coated on a substrate, dried at 50-90xc2x0 C., exposed to an active energy beam through a mask so that it is photocured in a patterned region. The resulting resist coating is developed by removing its unexposed region with the aid of an alkaline cleaner, and then exposed to heat at 140-170xc2x0 C. to increase a cure level of the previously photocured region.
Detailed explanation will be now given concerning the aqueous photosolder resist composition of the present invention which contains (A) an aqueous solution prepared by neutralizing a resin having a free-radically polymerizable group and a carboxyl group with a base; (B) an inorganic filler; (C) a photocurable mixture (C) containing (c-1) a multifunctional acrylic monomer, (c-2) a compound having a cyclic ether group other than a glycidyl group and (c-3) a photoinitiator; and optionally (D) an aqueous solution prepared by neutralizing a free-radically polymerized substance having an acid value of 130-230 mgKOH/mg with a base.
A first component incorporated in the aqueous photosolder resist composition of the present invention is obtained by neutralizing a resin containing a free-radically polymerizable group and a carboxyl group with a base. The resin contains at least one free-radically polymerizable group and at least one carboxyl group in a molecule and preferably has a free-radically polymerizable group content of 5xc3x9710xe2x88x924xe2x88x921xc3x9710xe2x88x922 mole/g and a carboxyl group content (i.e., acid value) of 30-200 mgKOH/g.
The free-radically polymerizable group content of below 5xc3x9710xe2x88x924 mole/g may lead to the insufficient cure level and accordingly to the reduced heat resistance of the resulting resist. If the free-radically polymerizable group content exceeds 1xc3x9710xe2x88x922 mole/g, an undesired proportion thereof may be left remained in the resulting resist. The carboxyl group content of below 30 mgKOH/g may result in the reduced alkaline developability. If the carboxyl group content exceeds 200 mgKOH/g, the dissolution of the exposed resist coating region may also result. It is more preferred that the free-radically polymerizable group content is in the range of 1xc3x9710xe2x88x923-5xc3x9710xe2x88x923 mole/g and the carboxyl group content is in the range of 30-150 mgKOH/g. The carboxyl group content is still more preferably in the range of 30-130 mgKOH/g, most preferably in the range of 70-110 mgKOH/g.
The aforementioned resin containing a free-radically polymerizable group and a carboxyl group preferably has a number average molecular weight within the range of 1,500-25,000. If the number average molecular weight is below 1,500, the insufficient heat resistance may be imparted to the resulting resist. If it exceeds 25,000, it becomes difficult for the resin to be dispersed in water.
The type of the aforementioned resin containing a free-radically polymerizable group and a carboxyl group is not particularly specified. Such a resin can be prepared, for example, by reacting a compound having plural epoxy groups with a compound having a free-radically polymerizable group and a carboxyl group, such as acrylic acid, and then allowing the hydroxyl groups produced as a result of the reaction to react with acid anhydride.
Various compounds can be used for the compound having plural epoxy groups. Examples of such compounds include aromatic polyepoxy compounds represented by the following structural formulas 1a-1c, as described in Japanese Patent Publication No. Hei 7-103213, and epoxides represented by the following structural formulas 2a and 2b and obtainable by reacting condensation products of phenols and phenolic hydroxyl-containing aromatic aldehyde with epihalohydrin, as described in Japanese Patent No. 2868190. In the formula 1, n is a value preferably in the range of 1-20. In the formula 2, m is a value preferably in the range of 0-15, more preferably in the range of 0-10. 
wherein Y1 and Y2 represent aromatic residual groups, respectively and independently; n is a number of 1 or higher; and M represents a hydrogen atom or an epoxymethyl group on conditions that, if n=1, M is an epoxymethyl group and, if n=2 or higher, at least one of M is an epoxymethyl group. 
wherein R is H or CH3 and m is a number of 0, 1 or higher.
Other suitable compounds having plural epoxy groups can be obtained by reacting a polymer derived from an acid anhydride-containing monomer with a compound having a free-radically polymerizable group and a hydroxyl group, such as 2-hydroxyethyl acrylate.
The resin having a radically-polymerizable group and a carboxyl group, when its carboxyl group is neutralized by the addition of a base, yields the aforementioned aqueous solution (A). The type of the base used is not particularly specified and may be selected from those widely recognized as neutralizing agents, examples of which include inorganic materials such as sodium hydroxide, potassium hydroxide and lithium hydroxide; and amine compounds such as triethyl amine, diethanolamine and diisopropylmethylamine. The amount of the base is suitably chosen depending upon the molecular weight of the particular resin containing a free-radically polymerizable group and a carboxyl group and its carboxyl content. The base may be added in the amount sufficient to render the resin water-soluble, e.g., in the amount to neutralize 30-200% of the carboxyl groups.
A second component incorporated in the aqueous photosolder resist composition of the present invention is an inorganic filler. The use of inorganic filler is to improve heat resistance of the resulting resist. Useful inorganic fillers are known in the art and include barium sulfate, finely powdered silicon carbide, amorphous silica and the like. Conventional flame-retardants are other examples of organic fillers. Dispersion of the inorganic filler can be achieved by adding directly to the above-described aqueous solution (A). For better dispersion, the inorganic filler may be rendered into an aqueous dispersion form before it is added to the aqueous solution (A).
The amount of the inorganic filler used is determined in terms of its proportion relative to a total solids weight of the aqueous photosolder resist composition of the present invention. This proportion will be discussed later in the description of the aqueous photosolder resist composition.
A third component incorporated in the aqueous photosolder resist composition of the present invention is a photocurable mixture containing a multifunctional acrylic monomer (c-1), a compound (c-2 )having a cyclic ether group other than a glycidyl group and a photoinitiator (c-3). The photocurable mixture exists in the form of being emulsified in the aqueous photosolder resist composition of the present invention.
The multifunctional acrylic monomer (c-1) has two or more polymerizable groups in a molecule. The incorporation of this multifunctional acrylic monomer (c-1) allows the composition to be photocured for the patterning purpose.
Examples of specific multifunctional acrylic monomers (c-1) include pentaerythritol triacrylate, dipentaerythritol pentaacrylate, polyethylene glycol diacrylate, polypropylene glycol diacrylate, trimethylolpropane triacrylate, penta-erythritol tetraacrylate, dipentaerythritol hexaacrylate and corresponding methacrylates. These monomers can be used alone or in any combination thereof.
The compound (c-2) having a cyclic ether group other than a glycidyl group, when heated, is caused to undergo a curing reaction with a carboxyl group of the resist composition. The inclusion of glycidyl group, among various cyclic ether groups, is not favored because of its tendency to deactivate in an aqueous medium. Preferred examples of cyclic ether groups are an alicyclic epoxy group and an oxetane group. Other cyclic ether groups may be introduced, for example, by oxidizing a substance having a double bond in its main chain, such as polybutadiene, or by allowing a compound having a vinyl-coupled alicyclic epoxy to undergo a ring-opening polymerization and subsequent oxidation. However, the use of the aforementioned two types is preferred since they are readily available in commerce.
Various types of alicyclic epoxy-containing compounds are sold in the market and include, for example, CELLOXIDE Series, EPOLEED Series and CYCLOMER Series, commercially available from Daicel Chem. Co., Ltd., and polymerizates thereof. Such compounds may contain the following structure in a molecule: 
The oxetane-containing compounds are also available in commerce, e.g., under the trade designations ALON-OXETANE Series from Toa Gosei Co., Ltd. The use of those containing two or more alicyclic epoxy or oxetane groups in a molecule, among the above-cited compounds, is preferred. The use of those containing three or more alicyclic epoxy or oxetane groups is particularly preferred.
Various photoinitiators conventionally known in the art can be used. Examples of specific photoinitiators include benzoin and benzoin alkyl ethers such as benzoin methyl ether, benzoin ethyl ether and benzoin propyl ether; acetophenones such as acetophenone, 2,2-dimethoxy-2-phenyl acetophenone, 2,2-diethoxy-2-phenylacetophenone and 1,1-dichloroacetophenone; aminoacetophenones such as 2-methyl-1-[4-(methylthio) phenyl]-2-morpholinopropanone-1,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,N,N-dimethyl aminoacetophenone; anthraquinones such as 2-methyl anthraquinonoe, 2-ethylanthraquinone, 2-tertiary-butyl anthraquinonoe and 1-chloroanthraquinone; thioxanthones such as 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, 2-chlorothixanthone and 2,4-diisopropylthioxanthone; ketals such as acetophenone dimethyl ketal and benzyl dimethyl ketal; benzophenones or xanthones such as benzophenone and 4,4xe2x80x2-bis(diethylamino)benzophenone; 2,4,6-trimethylbenzoyl-diphenylphosphine oxide and the like. These may be used alone or in any combination thereof. When needed, auxiliary photoinitiators may further be added, including ethyldimethyl-amino benzoate and tertiary amines such as triethanolamine.
The amounts of the multifunctional acrylic monomer (c-1), compound (c-2) having a cyclic ether group other than a glycidyl group and photoinitiator (c-3) incorporated in the photocurable mixture are determined in terms of their individual proportions relative to solids in the aqueous photosolder resist composition of the present invention. These proportions will be discussed later in the description of the aqueous photosolder resist composition.
Besides the aforementioned components, the photocurable mixture may further contain an amino-containing compound, such as triazine, which reacts with the cyclic ether group.
A fourth component optionally incorporated in the aqueous photosolder resist composition of the present invention is an aqueous solution prepared by neutralizing a free-radically polymerized substance having an acid value of 130-230 mgKOH/g with a base. The use of such an aqueous solution increases dispersibility of the inorganic filler. Examples of free-radically polymerized substances include copolymerization products of acidic monomers (carboxyl-containing monomers), such as acrylic acid, methacrylic acid, maleic anhydride and maleic acid, with acid group-free vinyl monomers such as carboxyl-free vinyl monomers. Useful free-radically polymerized substances have acid values (carboxyl contents) within the range of 130-230 mgKOH/g. The free-radically polymerized substance, if its acid value is below the specified range, reduces its ability to stably disperse inorganic fillers therein and, if its acid value is above the specified range, becomes almost incompatible with the aqueous solution (A) and photocurable mixture (C) to result in the failure to obtain a uniform coating film.
Preferably, the free-radically polymerized substance has a number average molecular weight in the range of 1,000-150,000. Its number average molecular weight, if below 1,000, may result in the reduced dispersibility of pigments, such as inorganic fillers, and, if above 150,000, increases a viscosity of the aqueous solution to an excessive level that may result in the difficulty to disperse inorganic fillers.
As stated above, the aqueous photosolder resist composition of the present invention contains (A) an aqueous solution obtainable by neutralizing a resin containing a free-radically polymerizable group and a carboxyl group with a base, (B) inorganic filler; (C) photocurable mixture comprising (c-1) a multifunctional acrylic monomer, (c-2) a compound having a cyclic ether group other than a glycidyl group and (c-3) a photoinitiator; and optionally (D) an aqueous solution obtainable by neutralizing a free-radically polymerized substance having an acid value of 130-230 mgKOH/mg with a base.
The aqueous photosolder resist composition of the present invention can be obtained by blending the above-listed components according to techniques well-known to those skilled in the art. For performance improvements of the resist composition and resulting resist, the components are preferably blended in such proportions that satisfy the following conditions.
A molar ratio (carboxyl/cyclic ether) of a carboxyl group content of the aqueous solutions (A) and (D) to a cyclic ether (exclusive of glycidyl) group content of the photocurable mixture (C) is preferably 1.3/0.7-0.7/1.3. If the molar ratio falls outside this range, the level of crosslinking will decrease, possibly leading to the insufficient heat resistance of the resulting resist. More preferably, the molar ratio is maintained within the range of 1.15/0.85-0.85/1.15.
A ratio (resin/free-radically polymerized substance) by weight of the resin in the aqueous solution (A) to the free-radically polymerized substance in the aqueous solution (D) is preferably 70/30-100/0. If the weight ratio exceeds 30%, delamination of a resist coating may be caused to occur during development.
The inorganic filler (B) is incorporated preferably in the amount of 5-60 parts by weight, based on 100 parts by weight of solids in the aqueous photosolder resist composition of the present invention. The amount of inorganic filler, if below 5 parts by weight, may result in the difficulties to impart sufficient heat resistance and to suppress shrinkage on curing and, if above 60 parts by weight, may result in the insufficient development. More preferably, it is maintained within the range of 15-35 parts by weight.
The multifunctional acrylic monomer (c-1) is contained in the photocurable mixture (C) preferably in the amount of 2.0-15.0 parts by weight, based on 100 parts by weight of solids in the aqueous photosolder resist composition of the present invention. If its amount falls below 2.0 parts by weight, insufficient photocuring may result. On the other hand, if its amount exceeds 15.0 parts by weight, the reduced heat resistance may result.
The photoinitiator (c-3) is incorporated in the photocurable mixture (C) preferably in the amount of 0.5-10 parts by weight, based on 100 parts by weight of solids in the aqueous photosolder resist composition of the present invention. If its amount falls below 0.5 parts by weight, a larger proportion of unreacted double bonds may remain. On the other hand, if its amount exceeds 10 parts by weight, the molecular weight may decrease excessively to result in the reduction in hardness of a resulting resist. The range within 1.0-8 parts by weight is more preferred.
The aqueous photosolder resist composition of the present invention may further contain a compound, such as melamine, that functions to increase a crosslinking level via its reaction with a hydroxyl group produced as a result of reaction between carboxyl and cyclic ether groups. The melamine may preferably be added in the amount of 40 or lower parts by weight, based on 100 parts by weight of the cyclic ether-containing compound (c-2).
Other than the above-described components, the aqueous photosolder resist composition of the present invention may further contain an internal solvent, when needed. This is employed to aid in the formation of a uniform film. Useful solvents have the ability to dissolve oil contents of the above-described components, boiling points preferably within the range of 120-250xc2x0 C., more preferably within the range of 135-200xc2x0 C., and hydrophilic properties. Examples of specific solvents include propylene glycol diacetate, dipropylene glycol monomethyl ether and propylene glycol monomethyl ether acetate. The internal solvent, if used, is added preferably in the amount of 20 or lower parts by weight, more preferably of 10 or lower parts by weight, based on 100 parts by weight of solids in the aqueous photosolder resist composition of the present invention. If its amount exceeds 20 parts by weight, the use of aqueous form for the purposes of obviating odor and safety concerns may become meaningless. The internal solvent may be added to any one of the above-described components, or formulated with those components into the aqueous photosolder resist composition.
The aqueous photosolder resist composition of the present invention may further incorporate a color pigment. Examples of useful color pigments are conventionally known in the art and include copper phthalocyanine, chlorinated copper pththalocyanine, quinacridone red and the like. The color pigment, if incorporated, may be present preferably in the amount of 0.02-4.0 parts by weight, based on 100 parts by weight of solids in the aqueous photosolder resist composition of the present invention. If the amount falls below 0.02 parts by weight, its coloring effect may become insufficient. On the other hand, if the amount exceeds 4.0 parts by weight, the photocurability decreases to the level that possibly causes delamination of the resist coating during development. The amount of 0.1-1.5 parts by weight is more preferred.
The aqueous photosolder resist composition of the present invention can be obtained by mixing the above-specified components, for example, according to the following procedures: The aqueous solution (A) is first prepared by neutralizing a resin containing a free-radically polymerizable group and a carboxyl group with a base. The inorganic filler (B) and an optional color pigment are added for dispersion in the aqueous solution (A) to which each of all components of (C), together with an optional internal solvent, are subsequently added to form an emulsion.
The aqueous solution (D), if used, may be mixed with the aqueous solution (A) prior to addition of the inorganic filler (B). The inorganic filler (B) is added and dispersed in the mixture to which each of all components of (C) is subsequently added to form an emulsion. Alternatively, the inorganic filler (B) may be added for dispersion in the aqueous solution (D) to which the aqueous solution (A) and then each of all components of (C) are sequentially added to form an emulsion.
The aqueous photosolder resist composition of the present invention, as such obtained in the emulsion form, contains dispersed particles having a surface mean diameter preferably of 0.1-10.0 xcexcm, more preferably of 0.3-2.0 xcexcm. It is substantially impractical to prepare an emulsion containing dispersed particles having a surface mean diameter of below 0.1 xcexcm. If the dispersed particles have a surface diameter of above 10.0 xcexcm, precipitation or flocculation thereof may be caused to occur.
The aqueous photosolder resist composition of the present invention preferably has a solids content of 20-70 weight %, more preferably of 35-60 weight %. The resist composition becomes economically disadvantageous when its solids content falls below 20 weight %. If its solids content exceeds 70 weight %, particles may be caused to fuse together and increase their sizes. A long-term storage of the aqueous photosolder resist composition of the present invention is assured if a mixture of the aqueous solution (A), optional aqueous solution (D) and inorganic filler (B) is stored separately from the photocurable mixture (C) before they are mixed together for actual use.
A solder resist film can be obtained according to the following procedures. The aqueous photosolder resist composition of the present invention is applied, in the form of a film, to a substrate, such as a board, dried generally at 50-90xc2x0 C., and then exposed to an active energy ray, such as an ultraviolet ray, through a mask. Development is achieved by removing the unexposed region with the aid of an alkaline cleaner. The photocured region is then exposed, for example, to heat at 140-170xc2x0 C. so that its cure level is promoted by heat cure.